1:  static void Main(string[] args)

   2:  {

   3:      string a = @"c:\a\b\c\d";

   4:      string b = @"c:\a\b\e\f\1.txt";

   5:      int n = 0;

   6:      Console.WriteLine(GetRelativePaths(b, a));

   7:      Console.ReadKey();

   8:  }

   9:   

  10:  public static string GetRelativePaths(string path, string current)

  11:  {

  12:      string a = current.ToLower();

  13:      string b = path.ToLower();

  14:      int i = 0;

  15:      for (; i < Math.Min(a.Length,b.Length); i++)

  16:      {

  17:          if (a[i] != b[i]) break;

  18:      }

  19:      string cur = Regex.Replace(a.Substring(i - 1), @"\\?[a-zA-Z]+:?", @"..\");

  20:      return (cur + path.Substring(i + 1)).Replace(@"\\", @"\");

  21:  }

..\..\f\1.txt